This invention relates to thermoplastic polyblends which offer both weatherability and impact strength and to a process for making such compositions.
Thermoplastic compositions with good impact strength are known and may be prepared by grafting vinyl monomers, such as styrene and acrylonitrile, onto a diene rubber, such as polybutadiene. There are sufficient unsaturation sites in the double bonds remaining in such dienes following polymerization of the diene to form the rubber to provide both crosslinking of the rubber and grafting of the vinyl monomers to the rubber. In polymers made with such diene rubbers, however, a problem occurs in that unsaturation sites remain in the polymer following polymerization which appear to result in poor weatherability of the polymer, as they are subject to attack by light and oxygen. Weatherability, as used in describing this invention refers to the ability to retain the composition's desirable physical properties, including its strength, on exposure to environmental effects such as high temperature, ultra-violet light, high humidity and rain and other elements generally included in discussions of weatherability.
Thermoplastic compositions with good weatherability are also known and are typically prepared by substituting a saturated rubber for the diene rubber. These compositions, however, lack strength when compared to ones made with a conjugated diene and having the same amount of rubber.
It is also recognized in the art that blending of two different particle size graft rubber copolymers, such as is taught in U.S. Pat. No. 3,509,237, provides a combination of high impact strength, good gloss, and good tensile properties.
It would be desirable in the art to have available a thermoplastic polyblend which would provide both good weatherability and strength.